In recent years, elastic fibers and an elastic nonwoven fabric are being used for such various purposes as a disposable diaper, a sanitary product, a clothing material, a bandage and a packaging material. In particular, a disposable diaper, a sanitary product and the like are used in direct contact with the skin and are demanded to have appropriate stretchability and elastic recovery property from the standpoint of comfort upon wearing on the body and mobility of the body after wearing.
As elastic fibers imparted with elastic recovery property, Patent Document 1 discloses elastic fibers obtained by mixing an elastomer, such as an olefin copolymer and a styrene block copolymer, and another resin component. However, the elastomer is poor in compatibility with polypropylene and is non-crystalline, and in the case where elastic fibers are formed by mixing the elastomer and polypropylene, the elastomer bleeds to the surface of the fibers. Accordingly, there are problems that an elastic nonwoven fabric formed of the elastic fibers has stickiness, and a fiber product using the elastic nonwoven fabric lacks pleasant texture.
Patent Document 2 discloses that a propylene polymer constituting a nonwoven fabric is treated with a free radical initiator. The treatment improves the fluidity of the propylene polymer but lowers the heat stability of the propylene polymer.
Patent Document 3 discloses that fibers are formed with a crystalline propylene polymer composition containing a crystalline propylene copolymer and a crystalline isotactic propylene homopolymer. However, the crystalline propylene copolymer is poor in compatibility with the crystalline isotactic propylene homopolymer as compared to a low crystalline polypropylene, and therefore, there are possible problems of poor kneading property and deterioration in property due to bleed on the surface of the fibers.
Patent Document 4 discloses fibers containing an olefin homopolymer having [mmmm] of less than 60%, but fails to disclose a mixing ratio with polypropylene with high regularity to have a problem in balance between elasticity and moldability.
Patent Document 5 discloses a resin composition containing a propylene polymer having (mmmm) of from 0.2 to 0.6 and [rrrr/(1−mmmm)]≦0.1, but the mixing amount of a low crystalline polypropylene is insufficient, and when the fibers are formed and pulled, the fibers are stretched but stay stretched. Accordingly, such a resin composition is demanded that provides a material that does not stay stretched but shrinks, i.e., fibers having elastic recovery property.
Patent Document 6 discloses that fibers are formed with a propylene composition containing propylene and ethylene, and that core/shell type composite fibers are formed with a propylene composition containing propylene and ethylene as a shell component and high density polyethylene as a core component. However, the propylene composition containing propylene and ethylene is not necessarily sufficient in compatibility with a crystalline isotactic propylene homopolymer, and therefore, there are possible problems of poor kneading property and deterioration in property due to bleed on the surface of the fibers.
[Patent Document 1]
JP-A-2003-129330
[Patent Document 2]
JP-T-2007-511680
[Patent Document 3]
JP-T-2001-520324
[Patent Document 4]
JP-T-2003-511578
[Patent Document 5]
JP-A-2003-27331
[Patent Document 6]
JP-A-2007-277755